Soil Bearing Capacity & How It Effects Your Structure

The bearing capacity of a soil is the allowable load that the soil can support before yielding (i.e. sinking/moving). When constructing buildings or houses, it is important for the structural engineer to understand the soil bearing capacity so that we can properly determine the size of the foundation. For a given load on a building foundation, if the soil is ‘weaker,’ the foundation footing must be larger to spread out the load; if the soil is ‘stronger,’ the foundation footing can be smaller.

For larger projects, a geotechnical engineer is involved early in the feasibility phase to study, test and verify the soil bearing capacity of a building site. Different soil compositions have different bearing capacities—even different areas of the same building site can have varying bearing capacities. A soil study is important for the safety of the building and will determine the most efficient type of foundation for a building. In the case of cast-concrete foundations, the soil bearing capacity determines how much concrete will be needed to support the building loads. Everyone involved in the project is impacted by the foundation as the project price will vary based on the volume of concrete.

For most residential projects, the loads are usually relatively low and a geotechnical engineer may not be needed. The structural engineer can assume a conservative soil bearing capacity and typically the building foundation can be supported by modest concrete footings. Sometimes it can be worthwhile for homeowners or contractors to hire a geotechnical engineer to determine soil bearing capacity because they might save money in concrete volume.

Low soil bearing capacity does not mean the end of a project, though. There are alternative foundation types (which we have written about in previous blogs) that can penetrate to deeper soils or spread the building loads. Structural fills can also be used to create better soil conditions. Constructing a building without a clear understanding of the soils can lead to bigger problems in the future such as settling, cracking, or tilting structures.

If you are planning a construction project and you are not sure about how to evaluate the soils on your building site, Team Engineering can help determine if your project should involve a geotechnical soil study, contact us if you have any questions.